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Problem getting vehicles to stick to the road in an F-Zero style game

I recently started working on a F-Zero style game with some friends and I had some problems with the vehicles. I followed the instructions on this post Creating a Magnetic Vehicles but it quickly starts flying when there is a downhill. The vehicle will also pass through the ground even if he's levitating like twice his size in height. Here is the script I use to make my vehicle move along the track: